1. Field of the Invention
The invention relates to an improved self-boosting electromechanical disk brake which is intended in particular for use as a wheel brake in motor vehicles.
2. Description of the Prior Art
One self boosting electromechanical disk brake known from German Patent Disclosure DE 101 51 950 A1 has a floating caliper of U-shaped cross section as its brake caliper, which fits over a brake disk on the circumference and in which two friction brake linings are located, one on either side of a brake disk. The brake caliper is supported displaceably transversely to the brake disk and is accordingly what is known as a floating caliper. However, the use of a fixed caliper is equally possible.
For actuation, the known disk brake has an electromechanical actuating device, with which a friction brake lining can be pressed against one side of the brake disk. As a result of the one friction brake lining being pressed against one side of the brake disk, the caliper is displaced transversely to the brake disk and presses the other friction brake lining against the other side of the brake disk, as a result of which the brake disk is braked. The electromechanical actuating device has an electric motor, an optional step-down gear, and a rotation-to-translation conversion gear. The most various kinds of gears may be used; besides gear drives, a rotatable cam as a rotation-to-translation conversion gear can for instance also be considered. Instead of an electric motor, an electromagnet can be used, for instance. In the generic terminology here, the adjective “electromechanical” accordingly refers to the type of actuation of the disk brake.
As its self-boosting device, the known disk brake has a wedge mechanism, with a wedge disposed on a back side, remote from the brake disk, of one friction brake lining; this wedge is braced on an oblique face in the caliper. The oblique face extends obliquely to the brake disk; there is a wedge-shaped gap between the oblique face and the brake disk that becomes narrower in a circumferential direction to the brake disk, specifically in the intended direction of rotation of the brake disk. If the friction brake lining is pressed against the rotating brake disk for actuation of the disk brake, then the rotating brake disk exerts a frictional force on the friction brake lining in the circumferential direction, and thus in the direction of the increasingly narrow wedge-shaped gap between the oblique face and the brake disk. Because of the principle of a wedge, the oblique face exerts a force on the friction brake lining, which includes a force component transverse to the brake disk. This force component transverse to the brake disk is a contact pressure, which presses the friction brake lining against the brake disk in addition to a contact pressure exerted by the actuating device. In this way, the contact pressure exerted by the actuating device is increased, and self boosting occurs. The wedge mechanism converts a frictional force, exerted by the rotating brake disk on the friction brake lining pressed against it, into a contact pressure of the friction brake lining against the brake disk. The wedge and the oblique face need not have a constant wedge angle over their entire length; the wedge angle may vary as a function of a displacement distance of the friction brake lining along the oblique face. In that case, the term ramp mechanism may be used. Preferably at the onset of tensing the disk brake, a large wedge angle or ramp angle is selected, for the sake of rapidly overcoming an air play between the friction brake linings and the brake disk. At the end of the tensing, when the braking force and contact pressure are high, a small wedge angle or ramp angle and consequently high self boosting are preferably selected.
To attain self boosting even for the reversed direction of rotation of the brake disk (travel in reverse), it is known to use a second wedge mechanism with an oppositely disclosed wedge and an oppositely extending oblique face. The wedge angle can differ for travelling forward and travelling in reverse, in order to attain self boosting actions of different strengths.
Other self-boosting devices are also known, for instance with one or more support levers that brace the friction brake lining, pressed against the brake disk, obliquely at a support angle to the brake disk. The support angle is equivalent to the wedge angle of the wedge mechanism.
The self-boosting disk brakes share the feature that the friction brake lining, for attaining the self boosting, is moved in the circumferential direction or the direction of rotation of the brake disk, and this motion, or the frictional force exerted by the brake disk on the friction brake lining pressed against it, is converted into a contact pressure of the friction brake lining against the brake disk. With decreasing thickness of the friction brake lining caused by wear, the displacement distance of the friction brake lining in the circumferential direction or the direction of rotation of the brake disk becomes greater. A displacement distance of the friction brake lining for attaining a defined braking force is lengthened as a result. This has the disadvantage that the time until a desired braking force is attained is lengthened. Another disadvantage is that a sufficiently large installation space for displacing the (worn) friction brake lining must be provided. In electromechanical disk brakes, in which some parts of the actuating device, of the floating caliper, or of a housing are displaced along with the friction brake lining, space problems can arise.